The present invention relates to silicone-based compositions adapted to use as working fluids for fluid coupling, more particularly, to a silicone-based working fluid for fluid coupling having an extended durability in output performance with little viscosity decrease by thermal degradation, as a consequence of the excellent heat stability thereof in both hermetically sealed and open conditions.
Fluid compositions containing a dimethyl silicone oil as the principal component are widely used as a silicone-based working fluid for fluid coupling in various types of transportation machines because of the outstandingly small decrease in viscosity accompanying temperature elevation and excellent heat-resistance thereof as compared with other mineral oils or synthetic oils.
Even such a fluid composition, however, has a disadvantage of viscosity decrease resulting from the rearrangement of the molecular chains at high temperatures of, for example, 200.degree. C. or above under influences of the internal friction in the oil during service, thereby causing a lowering of the output revolution from the fluid coupling. In addition, a gradual reaction by thermal oxidation of the dimethyl silicone oil by the oxygen in the coupling occurs on repeated use or for a long period of service, resulting in the phenomenon of viscosity increase eventually leading to gelation of the oil. The fluid coupling then falls into a condition of direct connection or seizure to give an output revolution exceeding the desired level, which sometimes leads to interruption of the revolution due to the overload. A great decrease in the revolution would be observed when the gelled structure is destroyed.